# Glossary

## Map elements

```{glossary}

Layer
   The main elements of a map are "layers". Layers can be imagined like a an ordered
   stack of transparencies. Each layer typically represents a single file or web service
   that provides map data. Layers can provide {term}`raster` or {term}`vector` data.

Raster
   A data {term}`Layer` composed of a regular grid of pixels. Conceptually, this is
   similar to an "image", but the data is often represented in measurement values, not
   color values.

Vector
   A data {term}`Layer` consisting of points, lines, and/or polygons.

Feature
   In a vector layer, a single point, line, or polygon element. For example, a single
   earthquake (point), a migration path of an animal (line), or a protected conservation
   area(polygon).

Attribute
   Additional data or measurement about a {term}`Feature` in a vector layer or grid cell
   in a {term}`raster` {term}`Layer`.

   For example, the temperature measured at a grid cell, earthquake magnitude on a point
   feature, the species of an animal on a line feature, or the area in square kilometers
   of a protected conservation area on a polygon feature.
```

## Symbology

```{glossary}

Symbology
   How data is {term}`visually encoded <visual encoding>` on the map.

Symbology Rule
   A set of {term}`Symbolizers <symbolizer>` that define a single representation of a data layer.
   Symbology rules are ordered; a higher rule will appear "above" a lower rule when
   rendered. A Rule can have a pre-processor, for example Kernel Density Estimation
   (KDE), which is applied before its Symbolizers.

Symbolizer
   A triple of data input, interpolation, and {term}`visual encoding`. A {term}`Rule
   <symbology rule>` is composed of many Symbolizers.

Visual encoding
   How data {term}`attributes <attribute>` are matched to visual characteristics like
   shape, color, or position ({footcite:t}`1984:cleveland-mcgill`, figure 1).

Color map
   A gradient or set of colors used to {term}`visually encode <visual encoding>` data
   with color on the map.

   Sometimes referred to as a "color ramp" or "color scale".

   **Important**: Color is a fraught {term}`visual encoding` method. Is your colormap
   [perceptually uniform](https://chrisholdgraf.com/blog/2018/makeitpop)? Is it usable
   by people with color vision deficiencies? How does it interact with the other
   elements on the map (for example, does your basemap include colors that interact
   poorly with your chosen color map for any form of human color vision)? Also see
   [understanding docs for WCAG SC 1.4.1 "Use of Color"](https://www.w3.org/WAI/WCAG22/Understanding/use-of-color.html).

Sequential color map
   A colormap which progresses between two colors. A sequential colormap is useful for
   representing an ordered scalar value like temperature or magnitude.

Divergent color map
   A colormap which diverges from a central value to extremes at each end. For example,
   a white center, dark red on one end, and dark blue on the other end. A divergent
   colormap is useful for representing scalar values that diverge from a critical value
   such as gain or loss (divergence from 0), or anomaly (divergence from average).

Cyclic color map
   A colormap which ends where it starts. A cyclic colormap is useful for representing
   cyclic values like phase or angle.

Categorical color map
   A colormap with a small set of discrete colors, without transitions between them. A
   categorical colormap is useful for representing categorical data, like land surface
   classification.

   Sometimes refered to as a "discrete" or "qualitative" color map, or a "color
   palette".
```

## References

```{footbibliography}

```